1. Field of the Invention
This invention relates in general to well casing handling equipment, and in particular to well casing elevator and spiders.
2. Description of the Prior Art
In a typical derrick arrangement, a traveling block is suspended from the derrick crown block by a series of cables, which are driven by the derrick drawworks to raise and lower the traveling block along the vertical axis of the derrick. The traveling block supports a derrick hook, a pair of links, and an elevator. When handling casing, sliptype elevators are used. Such elevators have a tapered interior bowl and a set of gripping slips, which are moved pivotally up and down within the bowl to grip the exterior surface of the casing.
A casing spider rests on the derrick floor and supports the casing string in the well bore with a set of slips, which are set to grip the casing exterior. A new joint of casing is raised into position over the well bore by the casing elevator, and the lower end of the casing joint is connected to the upper end of the casing string in the well bore. The elevator is then used to lift the casing string, releasing the slips of the lower spider, and then the casing string is lowered into the well bore. The slips of the spider are then set to support the casing string in the well bore, and the elevator is disengaged and stripped upward and off of the casing to allow another casing joint to be moved into position. This cycle is repeated until all of the casing has been run into the well bore.
Elevator/spiders are powerful, double-duty tools designed to handle long, heavy casing strings. These tools are convertible and can be used either as casing spiders or as elevators. Often, when handling casing strings, elevator/spiders will be used in tandem, utilizing one tool as a casing spider and the other tool as an elevator.
Elevator/spiders generally have slips which are pivotally operable between an upper, retracted position and a lower, gripping position. The slips are moved between the upper and lower positions by a yoke, which is connected to the slips by suitable linkages. The yoke pivots about a pivot axis in the approximate center of the yoke, when fluid pressure is applied to the fluid cylinders connected to the other end of the yoke.
The slips may also be raised and lowered manually by an operator using a handle inserted into a socket on the yoke. Whether the slips are raised and lowered manually or hydraulically, the weight of the slips should be counterbalanced. Counterbalancing the slips makes it easier for the operator to raise the slips, and lessens the chances of damage to the elevator/spider, the slips, and the casing, when the slips are lowered. U.S. Pat. No. 3,149,391 (Boster), issued on Sept. 22, 1964, shows one method of counterbalancing the weight of the slips. A compression spring is attached to the yoke to urge the yoke in a direction which applies an upward force on the slips. Another type of counterbalance is a torsional spring mounted on the pivot point of the yoke. All of the prior art counterbalances have been rather large, and somewhat difficult to install and to remove from the elevator/spider, partly because the spring must be preloaded prior to installment.